1. Field of the Invention
The present invention relates generally to frequency control circuits and methods and, more particularly, to frequency control of alternating current systems having individual reference oscillators. The present invention is applicable for matching the frequencies of parallel connected alternating current generators, and for adjusting the frequency of an AC motor in accordance with a frequency control signal.
The present invention has particular application in the field of variable speed constant frequency generator systems used in any aircraft environment. The present invention allows the generator systems to be connected in parallel so as to generate output signals having a frequency which is the average of all the individual frequencies of the generator systems. The present invention may also be used to control the prime mover speed in conventional generating systems so as to adjust frequency of the output of each machine when the outputs are connected in parallel.
2. Description of the Prior Art
The phase lock loop is a commonly used method of frequency control. In such phase lock loop systems, a master oscillator sets the system frequency, and all individual machines are then phase shifted with respect to this master oscillator in order to share the load. In this conventional method, the system maintains the master frequency. Such phase lock loop master oscillator systems work well during normal operation of parallel generator systems.
A major disadvantage of phase lock loop master oscillator systems is that if for any reason the master oscillator malfunctions, the entire system frequency will be lost. If this malfunction should occur in generator systems generating in parallel, such as a variable speed constant frequency (VSCF) system, there must be some backup means for providing a new master oscillator signal. When the master oscillator fails, however, logic circuitry must be provided to decide which of the oscillators of the other parallel systems is to become the new master oscillator for controlling the frequency of the entire parallel connected system.